Photosensitization: Reaction Pathways

Abstract

Thousands of naturally occurring and synthetic dyes can function as photosensitizers and inflict biological damage in the presence of light (Spikes, 1989 and references cited). The action is initiated by the absorption of a photon to yield an excited sensitizer and is followed by many competing dark reactions which ultimately result in the alteration of vital biomol-ecules. Reactions of the excited sensitizer can involve electron or hydrogen transfer, usually with a reducing substrate (Type I reaction) or interaction with oxygen (Type II reaction) (Foote, 1976 and references cited). The latter usually involves energy transfer to yield singlet molecular oxygen. Both Types I and Π pathways may compete, with the predominant route being determined by such factors as oxygen and substrate concentrations, the proximity of the sensitizer to the substrate as well as the nature of the sensitizers and the substrate. Both pathways ultimately lead to the formation of oxidized products and radical chain reactions resulting in extensive biological damage. Living organisms contain many enzymic and nonenzymic antioxidant mechanisms to protect against reactive oxygen species and the inability to control the latter has been defined as oxidative stress (Sies, 1986).